In a wireless communication system that employs a wireless transmission system such as orthogonal frequency division multiplexing (OFDM), signals of burst characteristics are often processed.
In general, an amplifier has amplifier operation points set to AB to B grades to improve power efficiency. Thus, the amount of current flowing to the amplifier during a no-signal period (during a burst-OFF period) is small. When a signal is suddenly input (burst ON) from the burst-OFF period, a current rapidly flows to an amplifier power source system. The power source system cannot follow such a sudden change in current value, causing current and voltage waveforms to exhibit transient responses (refer to FIG. 1). As a result, amplifier characteristics change.
As a related art of the present invention, techniques disclosed in the following two Patent Documents may be cited. First, Japanese Patent Application Laid-Open No. 2004-112252 (Patent Document 1) discloses a technique of adjusting an amplifier input signal to correct an amplifier distortion caused by burst transmission. This technique is characterized by measuring leading and trailing of a burst signal during transmission and performing a burst correction operation according to the leading and trailing thus measured.
Second, Japanese Patent Application Laid-Open No. H04-301950 (Patent Document 2) discloses a technique of restraining sudden rising to suppress burst distortion. This technique is characterized by suppressing the occurrence of burst distortion itself via ramp processing to gradually increase transmission outputs.
According to the technique described in Patent Document 1, a built-in correction coefficient is used, which does not follow the changes in amplifier characteristics. Because the amplifier undergoes changes in its characteristics due to a temperature or a passage of time, the amplifier has to maintain correction accuracy of burst distortion by adaptively updating the correction coefficient.
According to the technique described in Patent Document 2, a ramp operation is performed, which suppresses sudden rising of a transmission signal to gradually increase outputs. In this technique, a signal rising period has to be provided before burst-ON or a signal has to be gradually raised after burst-ON, which causes deformation of a waveform of an amplifier output signal.
The following are related arts to the invention.
[Patent document 1] Japanese Patent Laid-Open Publication No. JP 2004-112252
[Patent document 2] Japanese Patent Laid-Open Publication No. JP H04-301950
[Patent document 3] Japanese Patent Laid-Open Publication No. JP H09-153849
[Patent document 4] Japanese Patent No. JP 3560398